malloc memory to a pointer to pointer

Question

I have came across this problem when using pointer to pointer to a char:

void setmemory(char** p, int num)
{
    *p=(char*)malloc(num);
}

    void test(void)
           


        

Answer 1

You are only setting the local variable *p here. Remember you are getting a pointer to data, not a pointer-to-pointer-to-data.

Think of it like this:

First case:

int a;

foo(a); // Passes a
void foo(int b)
{
  b = 4;    // This only changes local variable, has no effect really
}

Second case:

int a;
foo(&a); // Passes *a

void foo(int *b)
{
  *b = 4; // This changes the contents of a. As you can see we have not changed the original pointer!
  b = 4; // This changes our local copy of the pointer, not the pointer itself, like in the first case!
}

Third case

int *ptr;
foo(&ptr); // Passes **ptr

void foo(int **b)
{
  **b = 4; // This changes the data of the passed pointer
  *b = 4; // This changes the passed pointer itself, i.e. it changes ptr. This is what test() is doing, the behavior you are looking for!
  b = 4; // This changes our local copy of a variable, just like the first case!
}